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Also, Americans' underlying puritanism and economically stressed lifestyle may make us more tolerant of sexual droughts in long-term relationships, reasons Tepper. "What I've seen in my life is that the frequency and quality of sex ebbs and flows, but the emotion of love remains consistent," says Robert G., 58, a New Hampshire entrepreneur who has been married 35 years. "My sense of what I need has matured to be less emphatic." Meaning the benefit of a long-term relationship may be more about stability than sack time—an observation supported by Rutgers University biological anthropologist Helen Fisher, Ph.D. Fisher's brain scans of people in long-term relationships showed increased activity in an area associated with calm, but none in a region associated with anxiety."

You can still feel in love and not be racing to the bedroom," she says. "Those periods come and go, but the intensity of connection remains."

The experience is a bit different—and the sex a bit spicier—in new relationships. Jim Mason, a widowed 69-year-old author from Virginia, recently found his soul mate "after many hits and misses"—and says that sex is crucial to their relationship. "Both of us get antsy without passionate sex."

And the sexual pleasure enjoyed by new 50-plus couples can be higher than for young lovers, says Amanda Barusch, Ph.D., author of Love Stories of Later Life . "Perhaps because new love late in life feels like more of a miracle, it can be more romantically intense," she says. In fact, American respondents ages 50 to 64 felt more strongly about sex as part of being in love—and were more likely to be in love—than the 18- to 24-year-olds surveyed. (The younger French, however, were more sexually driven than their 50-to-64-year-old counterparts.)

Brain scans show that love spurs the body to produce dopamine, a natural stimulant, whether you're 18 or 80. "We found activity in participants who had just fallen in love, and also in participants who reported they were in love long-term," says Fisher of Rutgers. "This activity was the same whether the individual was 18 or 50-plus. The body gets older, but the basic emotion—the need to be in love—remains the same."